1. Field of the Invention
The present invention relates to communication transceiver clock and data recovery, and, in particular, to tracking data in the presence of signal noise.
2. Description of the Related Art
In many data communication applications, serializer and de-serializer (SerDes) devices facilitate the transmission between two points of parallel data across a serial link. Data at one point is converted from parallel data to serial data and transmitted through a communications channel to the second point where it received and converted from serial data to parallel data.
At high data rates frequency-dependent signal loss from the communications channel (the signal path between the two end points of a serial link), as well as signal dispersion and distortion, can occur. As such, the communications channel, whether wired, optical, or wireless, acts as a filter and might be modeled in the frequency domain with a transfer function. Correction for frequency dependent losses of the communications channel, and other forms of signal degradation, often requires signal equalization at a receiver of the signal. Equalization through use of one or more equalizers compensates for the signal degradation to improve communication quality. Equalization may also be employed at the transmit side to pre-condition the signal. Equalization, a form of filtering, generally requires some estimate of the transfer function of the channel to set its filter parameters. However, in many cases, the specific frequency-dependent signal degradation characteristics of a communications channel are unknown, and often vary with time. In such cases, an equalizer with adaptive setting of parameters providing sufficient adjustable range might be employed to mitigate the signal degradation of the signal transmitted through the communications channel. An automatic adaptation process is often employed to adjust the equalizer's response. Equalization might be through a front end equalizer, a feedback equalizer (such as a decision feedback equalizer (DFE)), or some combination of both.
Further, SerDes devices are challenged by operation with very high insertion loss. Some insertion loss may be recovered through an analog front-end equalizer, sometimes in combination with a mixed mode DFE, but, even so, the receiver still operates with very low noise/jitter margin when equalized to a normal operating mode (in contrast to start-up or training modes). When the receiver has no margin at all, the receiver is inoperable, and the receiver must retrain to the input signal. Upon retraining, the receiver determines if the input data is a spectrally rich data stream (i.e., nearly random in contrast to fixed patterns). Once a spectrally rich data stream is present, the equalizer might implement an automatic adaptation process to adjust equalizer coefficients and, thus, the equalizer's response.